This invention relates to an optical rotary encoder for measuring an angle of rotation. More particularly, the invention relates to an optical rotary encoder adapted for torque detection and provided in the transfer of a four-wheel drive vehicle equipped with a center differential mechanism.
When an automotive vehicle is traveling, front-wheel drive generally provides better stability for forward motion than rear-wheel drive. When the vehicle is cornering, however, the tires attempt to return to their original attitude and a force must be applied to them by turning the steering wheel. Turning tends to be difficult with front-wheel drive. Though turning is easier with rear-wheel drive, a driving force which is too strong can result in an excessive turn. Accordingly, applying a driving force evenly to the front and rear wheels is ideal for stable vehicle travel. This is achieved with excellent results with a four-wheel drive vehicle.
The left and right wheels of an automotive vehicle exhibit different turning radii at cornering. In order to compensate for the effects of this phenomenon and achieve cornering smoothly, a four-wheel drive vehicle is equipped with a differential mechanism (front and rear differentials) which absorbs the difference in rotating speeds between the left and right wheels in dependence upon the difference in turning radius. Since a difference is turning radius also develops between the front and rear wheels, it has been proposed to provide a four-wheel drive vehicle with a center differential mechanism which absorbs the difference in rotating speeds between the front and rear wheels in dependence upon the difference in turning radius.
However, since the center differential mechanism functions to distribute front and rear wheel torque at an equal ratio, a limitation upon the transfer of driving force results in balance being achieved at either the front-wheel or rear-wheel driving force, whichever has the lower value. For example, if one of the front wheels should happen to lose traction with the road and begin racing, the driving energy will dissipate itself at the front wheels so that very little driving force will be applied to the rear wheels. Consequently, there are situations where a four-wheel drive vehicle with a center differential mechanism exhibits a deterioration in transmitted driving force, as when the road surface has a low coefficient of friction, in comparison with a four-wheel drive vehicle without a center differential mechanism. This can cause a phenomenon such as slipping (racing) of the front or rear wheels due to an inability to transfer the driving force to the road surface sufficiently, as when a large driving force is produced at acceleration.
In order to prevent these detrimental effects, the conventional practice is to provide a locking mechanism for a direct transfer of power between the front and rear wheels without the intervention of the center differential. When a large driving force becomes necessary, as during acceleration or when driving on a poor road surface, the center differential is locked manually. Under ordinary driving conditions when a large driving force is not required, the center differential is manually unlocked.
If the vehicle is traveling with the center differential locked, however, at cornering the front wheels will rotate too fast in comparison with the rear wheels when the turning radius is small. As a result, a negative torque develops on the front-wheel side and it is just as if braking were being applied solely to the front wheels. In other words, a so-called "tight corner braking" phenomenon occurs. If the vehicle is traveling at high velocity and the centrifugal force produced by the turn is large, this phenomenon can cause the tires to skid in the centrifugal direction so that the difference in rotating speed between the front and rear wheels is absorbed by the skidding of the tires. This has a deleterious influence upon the stability of the traveling vehicle during cornering. Manually unlocking the center differential mechanism is not a solution because this will not allow the condition of the road surface to be judged accurately.
Thus, if the center differential mechanism is locked during vehicle travel, the tight corner braking phenomenon occurs. In order to prevent this with assurance, it is necessary to monitor front wheel torque constantly and unlock the center differential mechanism automatically if, say, the front wheels develop a negative torque. This makes it necessary to measure the torque of the driven wheels. One method of accomplishing this is to measure the angle of torsion of the drive shaft.
One known expedient for measuring a turning angle such as the torsional angle of a shaft is an optical rotary encoder.
FIG. 10 illustrates an optical rotary encoder according to the prior art, and FIG. 11 is a block diagram of a turning angle measurement apparatus using an optical rotary encoder.
In FIG. 10, the rotary encoder is shown to include slitted disks 1, 2 each of which is provided with circumferentially spaced slits. Specifically, the disk 1 is provided with slits 3, and the disk 2 is provided with slits 4, 5 staggered by one-quarter of a pitch with respect to the slits 3. Numerals 6, 7 denote light-emitting elements, and numerals 8, 9 represent light-receiving elements. In FIG. 11, the output sides of the light-receiving elements 8, 9 are connected to the input sides of amplifier circuits 10, 11, respectively. Comparator circuits 12, 13 compare the respective outputs of the amplifier circuits 10, 11 with a fixed reference voltage and produce an output when the output from the corresponding amplifier circuit exceeds the reference voltage. The outputs of the comparator circuits 12, 13 are applied to output circuits 14, 15, respectively.
When the slitted disks 1, 2 rotate relative to each other, the light-receiving elements 8, 9 produce outputs displaced from each other by 90.degree.. The direction of rotation is judged based on which of the phases is leading. These outputs from the light-receiving elements are compared with the fixed reference voltage by the comparator circuits 12, 13. When the outputs exceed the reference voltage, the output circuits 14, 15 produce output pulses. The relative turning angle is measured by integrating these output pulses.
Ordinarily, an optical rotary encoder of this type is enclosed in a sealed package-like container and is used in a clean environment where it will not be splattered by oil, grease or the like. Accordingly, when it is attempted to measure the turning angle of a vehicle drive shaft by installing the optical rotary encoder in the transmission or the like and setting up the measurement apparatus shown in FIG. 11, the encoder must be immersed in oil. When the ability of the oil to transmit light diminishes due to temperature or, above all, contamination, the light-receiving elements 8, 9 receive less light. This not only makes it difficult to measure the turning angle accurately but can make measurement impossible in extreme cases.
FIG. 12 is a view useful in describing such a situation. In FIG. 12, a.sub.1, b.sub.1 illustrate the outputs of the respective light-receiving elements 8, 9 when the rotary encoder operates in the air. Similarly, a.sub.2, b.sub.2 show these outputs in clean oil, and a.sub.3, b.sub.3 show them in contaminated oil. The waveforms c.sub.1, d.sub.1 indicate the outputs of the respective comparator circuits 12, 13 when the encoder operates in air, and the waveforms c.sub.2, d.sub.2 indicate these outputs when the encoder operates in clean oil. A level e is the fixed reference voltage applied to the comparator circuits 12, 13, and a level f represents the light-receiving element output when the light from the light-emitting elements is blocked off completely from the light-receiving elements.
When the rotary encoder is disposed in the air, an adequate amount of light reaches the light-receiving elements. As a result, the outputs a.sub.1, b.sub.1, which differ in phase by 90.degree., are obtained from the light-receiving elements 8, 9, these outputs are compared with the fixed reference voltage e, the output pulses c.sub.1, d.sub.1 are obtained from the comparator circuits 12, 13, and these pulses are integrated to measure the angle of torsion. When the rotary encoder is operated in clean oil, the light-receiving elements receive less light than they do in air. This means that the outputs of these elements will decrease to a.sub.2, b.sub.2. When these outputs are compared with the fixed reference voltage e, the result is that the output pulses produced by the comparator circuits have a narrower pulse width, as indicated by c.sub.2, d.sub.2. If the rotary encoder is immersed in contaminated oil, the light reaching the light-receiving elements a.sub.3, b.sub.3 diminishes even further, so that the outputs from these elements fall below the fixed reference level 3, as indicated by a.sub.3, b.sub.3. Consequently, output pulses can no longer be obtained from the comparator circuits 12, 13, thus making measurement impossible.